Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane or PEM to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system.
In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; these collectively transport the various reactant gases flowing through each cell. Gas diffusion media or assemblies then distribute these fluids from the flow field to the reactive anode and cathode focus of a membrane electrode assembly or MEA. These gas diffusion media are frequently advantageously formed as a part of the design of primary collector electrodes pressing against the reactive anode and cathode faces.
Effective operation of a PEM requires a balanced provision of sufficient water in the polymer of a PEM to maintain its proton conductivity even as the catalyst adjacent to the PEM, the flow field, and the gas diffusion media are maintained in non-flooded operational states. In this regard, the oxidant, typically oxygen or oxygen-containing air, is supplied to the cathode where it reacts with hydrogen cations that have crossed the proton exchange membrane and electrons from an external circuit. Thus, the fuel cell generates both electricity and water through the electrochemical reaction, and the water is removed with the cathode effluent, dehydrating the PEM of the fuel cell unless the water is otherwise replaced. It is also to be noted that airflow through the cathode flow field will generally evaporate water from the proton exchange membrane at an even higher rate than the rate of water generation (with commensurate dehydration of the PEM) via reaction at the cathode.
When hydrated, the polymeric proton exchange membrane possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the proton exchange membrane is not sufficiently hydrated, the “acidic” character diminishes, with commensurate diminishment of the desired electrochemical reaction of the cell.
A problem, however, in membrane hydration occurs in operation of the fuel cell as moisture mass transfer within the cell establishes localized moisture gradients in gas diffusion media. In this regard, an imbalance within the plane of the proton exchange membrane interfacing to the gas diffusion media occurs as some areas in the plane of the membrane benefit from a higher level of moisture respective to other areas of the plane in operation. The localized imbalances in gas diffusion medium moisture quality effect comparable differentiated moisture qualities in localized areas of the proton exchange membrane resulting in differentiated efficiencies per local areas of the proton exchange membrane in generation of electricity from the cell.
Another complexity in membrane hydration is that many fuel cell catalysts are deactivated when saturated with liquid water. As a result, solutions to providing balanced hydration across the plane of the proton exchange membrane are also constrained respective to the negative impact of liquid water on the activity of the catalyst adjacent to the surfaces of the proton exchange membrane when the catalyst itself becomes hydrated to saturation with liquid water either locally or across the plane of the PEM to which the catalyst is adjacent and/or attached.
What is needed is a fuel cell power system providing comprehensively balanced hydration of the proton exchange membrane along with maintenance during operation of full activity in the catalyst attached to the surfaces of the proton exchange membrane. The present invention is directed to fulfilling this set of needs.